Another option would be to use the car park at the Odessey Arena. It costs about £1 per hour up to 4hours and then reduces slightly thereafter.
The walk from the car park to the City Hall is about 2km so will take 30/40 minutes depending on how you hit the lights at the pedestrian crossings.
The bus from Sprucefield takes about 30minutes during rush hour and 20 minutes the rest of the day. It is about a 10minute walk from Belfast Europa bus station to the City Hall. Bus frequency is variable but typically every 20/30 minutes, but sometimes longer in middle of the day. Worth checking on the Translink website. Bus stop name is Blaris, Sprucefield P&R